Zeolite A LTA

An interesting contribution to infrared investigations of frameworks is contained in the review article by Stein and Ozin [257] on sodalite superlattices. These authors determined the effect of the number of Na4Br clusters on the far-infrared spectra of dehydrated sodalites of the type Na8 pBr2.p [ p, where [ stands for an anion-free sodalite cage and p runs from 2 to 0. 

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Small Pore Zeolites Zeolite A (LTA) 4.1 A diameter pore, 11.4 A diameter cavity... 

Figure 4.2 Zeolite cages as found in Sodalite (SOD), Zeolite A (LTA), and Faujasites (FAU). (Reprinted from D.

A broad range of high silica and pure silica molecular sieves have been synthesized by employing hydrothermal synthesis in fluoride media at low H2O concentration, near neutral pH and alkali-free [81]. The significant new pure silica zeolite, lTQ-29, a structural analog of zeolite A (LTA) was reported by Corma at al [82]. Unlike the highly hydrophilic zeolite A, the ITQ-29 is hydrophobic. 

FIGURE 7.6 Zeolite frameworks built up from sodalite units (a) sodalite (SOD), (b) zeolite A (LTA), and (c) faujasite (zeolite X and zeolite Y)... 

FIGURE 7.8 Framework and cation sites in the Na form of zeolite A (LTA). See colour insert following page 356. (Courtesy of Dr. Robert Bell, Royal Institution of Great Britian, London.)... 

FIGURE 7.10 Computer model of the zeolite A (LTA) structure, illustrating the channels and 8-ring windows. 

The zeolites were synthesized in one of six reaction series. Reactant ratios for the six series are listed in Table I. Series 1 is one that we have used, in the absence of TMA, to synthesize zeolite Y (FAU). Series 2 is based on a published recipe(7) for synthesis of ZK-4 (LTA). Series 3 is one which we use to synthesize zeolite A (LTA) at the low silica end of the range used here. Series 4 and 5 are Series 3 with added TMAOH or NaOH each increases the pH approximately equally. Series 6 is based on a published recipe(9) for synthesis of zeolite HS. In Series 1, 2, and 6, substitution of TMAOH for NaOH causes only minor increases in pH. 

Figure 3 Framework of synthetic zeolite A (LTA) showing constituent cage structures and their dimensions...

Detergency. The largest-scale industrial production of zeolites is that devoted to the production of the sodium form of zeolite A (LTA) for use in the detergent industry. This uses currently in excess of 0.5 million metric toimes per annum worldwide. Zeolite A (NaA) is added to washing powders and other household detergents or cleansing powders as a water softener (described by the detergent industry as a builder). It... 

Figure 19 Pore apertures of DOH (6-membered ring. 0.28 nm), zeolite-A (LTA, 8-membered ring, 0.41 nm), silicalite-1 (MFI, 10-membered ring, 0.52 X 0.55 nm), and a zeolite-X or -Y (FAU, 12-membered ring, 0.74 nm) and a methane molecule (kinetic diameter = 0.38 nm).

Alkalinity also has an important influence on the crystallization rate of zeolites. A remarkable example is the crystallization of zeolite A (LTA) from the precursor gel with a batch composition of 5Na20 Al203 2Si02 (100-200) H20.[13] Figure 3.8 shows the effect of different alkalinity (H2O/Na2O=20, 30,40) on the crystallization rate (including induction period and growth rate) and particle size of the product. Clearly, with an increase of alkalinity the crystallization process is speeded up, the particle size is decreased, and the distribution of the particle size is narrowed due to an increased nucleation rate and an increased polymerization rate between polysilicate and aluminate anions. 

Figure 9.6 Cluster crystals formed by Na43+ clusters located in the sodalite cages of zeolites with different structures, (a) Sodalite (SOD), (b) Zeolite Y (FAU). (c) Zeolite A (LTA). Although the array shown in (c) has not been prepared, the analogous potassium clusters shown in (d) is indeed available in (d) the cluster crystal is actually composed of K43+ clusters in all the sodalite cages and the larger K,24+ clusters in every other a-cages. Reproduced from [4], Copyright (2002) Springer-Verlag...

If there are two or more types of T-atoms and these are ordered (i.e. not randomly distributed over all T-siles), the ideal symmetry of the framework type is likely to be reduced. For example, Al and Si alternate in the framework structure of zeolite A (LTA). To illustrate the effect of this ordering on the symmetry, the LTA framework type with all nodes identical and with alternating nodes marked are shown in Figs. 20a and b, respectively. The lattice constant (repeat distance) a and one of the mirror planes for the former is shown in Fig. 20a. In Fig. 20b, the symmetry reduction dictated by the ordering of Si and Al is readily apparent. Two obvious effects of the alternation are that (I) the mirror planes between sodalite cages arc gone, and (2) the unit cell has to be doubled along each of the axes. Similar elfects are observed in other materials in which the T-aloms are ordered. 

Fig. 21. The 8-ring in zeolite A (LTA) showing a Na+ ion position (gray) with its bonding to 3 framework oxygens, and the 3 unoccupied symmetry equivalent Na+ ion positions (dotted circles).

Similarly, zeolite A (LTA) was obtained in 2 and 4 h, respectively, with and without the use of promoter (Na3P04). However, the effect of promoter is more pronounced in the... 

Figure 2.4 The construction of zeolite frameworks (a) directly connected sodalite cages in the mineral sodalite (b) zeolite A (LTA), with oxygen bridges between square ports (c) zeolite X (or Y) (FAU), with oxygen bridges between hexagonal ports.

Many zeolites have more than one pore structure, and Table 2.1 lists the dimensions of the largest pores in nanometres. These pores can be circular, as in zeolite A (LTA) with a diameter of 0.41 nm, whereas others are elliptical, as in erionite (ERl B) with dimensions 0.36 x 0.51 nm. As well as having different size circular or elliptical pores, zeolites may also have pores that run in just one dimension, or in two or three dimensions. We can see that with this wide range of pore sizes and orientations, the separation of various substances with different molecular sizes can be achieved. 

The structure of zeolite A (LTA) (a) line drawing (b) eight-membered ring cross-section viewed along the direction of the arrow in (a) (c) schematic representation of the pore system. 

The CD-ROM associated with this book contains several structures of zeolites that can be viewed using WebLab ViewerLite erionite (ERI), faujasite (FAU), zeolite A (LTA), mordenite (MOR), ZSM-5 (MFI) and ZSM-11 (MEL). 

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